The present invention relates to photoconductive materials and to methods for their production, more particularly the invention relates to the polymerization of monomers or prepolymers to yield photoconductive polymers.
Currently used electrographic materials consist of a support and a photoconductive layer containing as photoconductor selenium or zinc oxide. Organic substances such as anthracene, benzidine or certain types of heterocyclic compounds are known to be used instead of the abovementioned inorganic materials.
Electrostatic recording materials have also been reported. These consist of a photoconductive layer such as antimony trichloride covered with an insulating layer of polymer such as polystyrene.
It is also known that certain polymers can also retain in the dark a high electric discharge for a period of time, but lose this charge on exposure to electromagnetic radiation of suitable wavelength.
In all of this prior art, adhesion of the photoconductive material to the surface of a substrate can be a severe limitation to the application of the technique. Some of the known photoconductive polymers are difficult to coat onto suppports due to their high molecular weight. It is also time consuming to prepare the polymer in a separate step.
The present invention consists in a photoconductive medium comprising a substrate having adhered to a surface thereof a layer of a polymeric material formed in situ on the substrate by the complete polymerization of one or more monomers selected from the group having the formula: ##STR1## wherein A is a vinyl group, a hydrogen atom or an alkyl group containing up to 7 carbon atoms and R1, R2, R3, R4, R5, R6, R7 and R8 are each selected from the group comprising a hydrogen atom, a halogen atom and a vinyl group, with the proviso that when A is a hydrogen atom or a lower alkyl group at least one of R1, R2, R3, R4, R5, R6, R7 and R8 is a vinyl group, or by the complete polymerisation of one or more prepolymers formed by the partial polymerisation of one or more of the said monomers.
In a further aspect the present invention consists in a method of rendering a surface of a substrate photoconductive, comprising the steps of applying to the surface a layer of one or more monomers selected from the group having the formula: ##STR2## Wherein A is a vinyl group, a hydrogen atom or an alkyl group containing up to 7 carbon atoms and R1, R2, R3, R4, R5, R6, R7 and R8 are each selected from the group comprising a hydrogen atom, a halogen atom and a vinyl group, with the proviso, that when A is a hydrogen atom or a lower alkyl group at least one of R1, R2, R3, R4, R5, R6, R7 and R8 is a vinyl group, or applying to the surface a single layer of one or more prepolymers formed by the partial polymerisation of one or more of the said monomers, and polymerising the monomer(s) or prepolymer(s) completely in situ on the surface of the substrate.
For the purpose of this invention, grafting can be achieved by thermal, free radical or radiation-initiated means. Suitable radiation sources include ultra violet lamps and ionizing radiation sources such as cobalt-60, electron accelerators and Van der Graaff Generators. Ultraviolet initiation is particularly attractive since, with a suitable lamp, the polymerisation is completed within a fraction of a second and thus it is feasible to include a U.V. source within a Xerographic or Xeroradiographic machine to polymerise a monomer coated onto a support within the machine and to simultaneously graft the polymer to the support. "Imaging" being achieved in a subsequent step within the same machine.
As a guide to "imaging" properties of the present polymers, the ratio of the photoconductivity to the dark current is useful. There appears to be a threshold value which must be achieved to attain "imaging". Above this threshold value, any increase does not appear to affect the "imaging". The fact that finite photoconductivity dark current ratios can be induced into supports by the present technique is also of interest since, although such values may not reach the threshold required to give imaging, the induced property can be of use in some applications.